The present invention relates to the general field of fuel injectors in turbomachines and it relates more particularly to assembling injectors for a two-headed combustion chamber in such a turbomachine.
In two-headed combustion chambers, a turbojet or a turboprop (referred to below in the present description as a xe2x80x9cturbomachinexe2x80x9d) is started and kept idling using so-called xe2x80x9cpilotxe2x80x9d injectors, while xe2x80x9cmainxe2x80x9d injectors are used only when cruising. Pilot injectors are fed with fuel on a permanent basis whereas main injectors are fed only once the turbomachine is rotating at more than some minimum determined speed (generally lying in the range 10% to 30% of its nominal speed). Furthermore, during so-called xe2x80x9cstage-burningxe2x80x9d, only half of the main injectors are in operation, with the other half of the main injectors then being temporarily stopped.
Various types of injector architecture are known. Thus, international patent application WO 94/08179 shows a conventional two-headed structure for which the main injector is shown in FIG. 3 and the pilot injector in FIG. 4. Each of those two injectors is essentially characterized by a terminal portion including a large number of parts and requiring sealing gaskets to ensure that the primary and secondary circuits are sealed from each other.
This results firstly in such injectors being complex to manufacture and assemble, and secondly, under certain operating conditions and in particular at high temperatures, in performance being degraded because of a considerable reduction in the lifetime of the combustion chamber and/or of the turbine, or indeed because of destruction of the injector and corresponding destruction of the turbomachine.
The present invention provides a method of assembling the end portion of an injector which mitigates the above-mentioned drawbacks. An object of the invention is thus to make the end portion with a minimum number of parts and in a small space. Another object of the invention is to integrate a cooling circuit in this end portion of an injector so as to enable the injector to be used at very high temperature.
These objects are achieved by a method of assembling the end portion of an injector for a turbomachine combustion chamber, the injector comprising means for delivering primary fuel, said means comprising a first feed tube connected to an injection piece having first injection orifices for discharging the primary fuel into said combustion chamber, and means for delivering a secondary fuel, said means comprising a second feed tube surrounding said first tube and connected to a cylindrical endpiece surrounding said annular injection piece and having second injection orifices for discharging the secondary fuel into said combustion chamber, wherein said cylindrical endpiece and said annular injection piece are provided with radial wells for receiving a brazing metal, and said wells are initially filled with said brazing metal; thereafter said annular injection piece is fitted in said cylindrical endpiece and these pieces are together fitted on said first and second tubes for feeding primary and secondary fuel, and to an outer wall of the injector; finally the end portion of the injector as assembled in this way is placed in an enclosure where it is heated so as to cause the brazing metal to melt and unite the parts.
By using this brazing technique, assembly of the end portion of an injector is made considerably simpler, and very reliable, while also being accelerated. Furthermore, the very small number of parts required for making such an injector end portion (only two parts fitted to the ends of the feed tubes in the preferred embodiment) considerably facilitates subsequent maintenance.
Advantageously, said annular injection piece is fitted on said first feed tube via a cylindrical connection piece including radial wells for receiving the brazing metal. Adding this third part enables the machining of the annular injection piece to be simplified and facilitates possible replacement thereof.
In an embodiment more particularly intended for assembling a main injector, prior to fitting said outer wall of the injector, a separator wall is fitted in said cylindrical endpiece, with a downstream end of said wall being fixed on a third tube for delivering a cooling fluid surrounding said first and second feed tubes.
The brazing metal is preferably based on gold or nickel, and the enclosure is raised to a determined temperature lying in the range 600xc2x0 C. to 1100xc2x0 C. as a function of the nature of the parts to be assembled together and of the brazing metal used.
The present invention also provides a terminal portion of a fuel injector for a turbomachine combustion chamber made using the above-specified brazing assembly method.